While human beings embody a unique ability for planned behaviour, they also often act automatically. In this study, we draw on the automatic thinking perspective as a meta‐theoretic lens to explain why online auction bidders succumb to both trait impulsiveness and sunk cost, ultimately leading them to experience winner's regret. Based on a survey of 301 online auction participants, we demonstrate that both trait impulsiveness as an emotional trigger and sunk cost as a cognitive trigger promote winner's regret. By grounding our research model in the automatic thinking view, we provide an alternative meta‐theoretical lens from which to view online bidder behaviour, thus bolstering our current understanding of winner's regret. We also investigate the moderating effects of competition intensity on the relationships between the triggers of automatic thinking and winner's regret. Our results show that both trait impulsiveness and sunk cost have significant impacts on winner's regret. We also found that the relationship between these two triggers and winner's regret is moderated by competition intensity.